The light loess soils of Northwest China are easily shaken by an earthquake. In 1556, a huge population lived there in underground homes and many were killed by this, the world’s deadliest earthquake.
The Shaanxi earthquake of 1556 was the deadliest ever experienced anywhere in the world. It happened in the northern interior of China not far from the city of Xi’an. It measured more than 8 on our scale of earthquake strength, damaged parts of ten neighboring provinces in addition to the Shaanxi one, and was so strong that half of China felt its impact. Aftershocks continued intermittently for six months. An area extending 250 miles outward from the epicenter was completely destroyed. Most of the population at that time lived in caves inside the loess cliffs. Loess is a relatively light type of soil and it disintegrated under the earthquake’s pressure and shaking with the result that 830,000 people were killed.
In 1989, the Science Press of Beijing, with help from UNESCO, published a summary of the history of Chinese earthquakes. Its description of the 1556 one tells of mountains and rivers changing places and roads being completely destroyed. In several places earth movements pushed the surface up sufficiently to form new hills. At the same time, these same areas had sections that collapsed to form new valleys. Streams and gullies appeared in new locations. Overall, huts, official houses, temples, and city walls generally collapsed suddenly. Numerous valuable monuments were destroyed beyond repair. The important Small Goose Pagoda in Xi’an withstood the shaking but its height dropped by about six feet. The report from Beijing dealt with earthquakes of all kinds. It covered the long history of China and demonstrated for the first time the constant threat that the nation always had to face. Ten earthquakes like the 1556 one were listed, each of magnitude 8 or more, and of these the largest cluster appeared in areas close to the city of Xi’an.
The Loess region is of great historic importance to the Chinese. It is the cradle of civilization for the country. In ancient times it was the meeting zone between the nomadic herders of the Mongolian and central Asian lands and the more settled farming communities of what would one day become China. It was on the margins of these loess areas that the great trading cities and capitals of empire first formed. Xi’an is one of the best known of these. Today it is a city of the same size as Chicago. In the 1930s and 1940s, fully aware of its historical significance to China, Mao Zedong and the Chinese Communist Party set up their headquarters here and made preparations for their conquest of China which happened in 1949. Today, some two hundred million people live in or near the loess lands with almost one quarter of them living in cave housing. The importance of this part of China is seen in the Great Wall of China, built to protect the high concentrations of people to the south of it from the marauding tribes to its north. The Mongol armies of eight hundred years ago were the strongest of these.
Loess is wind-born silt that has settled out over many thousands of years, frequently to a depth of three hundred feet in provinces in and around Shaanxi. Much of this loess accumulated in the aftermath of the last ice age as winds from the Mongolian Plateau picked up and transported tiny particles of rock from the barren land surfaces left by the ice. It has a yellowish to light brown color and has no structure of layering like sedimentary rocks. As a result, it breaks away easily whenever it is cut away by stream action, leaving high cliffs alongside the stream. These cliffs provide ready access for excavating cliff-side homes and people have taken advantage of this opportunity for thousands of years both in China and in parts of Central Europe. Caves provided warmth in winter and cool conditions in summer. The climate of Shaanxi is semi-arid with an annual rainfall of twenty inches. However, this amount of rain in some years may fall in a single day and in loess territory that can be catastrophic because the soil is easily eroded and washed away. Its lack of layering means it has no natural cohesion. Frequent landslides occur with heavy rain and they cause considerable loss of life.
The Hwanghe, or Yellow River as it is often named because of the load of yellow loess particles that it always carries, cut its channel over time through these mountains of loess. In its upper reaches, because of the interior location, occasional bursts of rain are experienced in the warmer months of summer and the river overflows the levees that hold the flow of water to its main channel. Farms get destroyed when this happens. The usual reaction in past times was to have thousands of workers rebuild the levees by painstaking hard manual work. It is very difficult to predict these overflows because of the large volume of yellow silt that the river carries. A buildup of this silt at a turning point in the river can quickly become cumulative as the silt slows down the flow of water, thus speeding up the rate at which silt is deposited. Loess is an excellent parent material for the formation of rich dark soils so these soils coupled with plentiful supplies of water from the river gave rise to the earliest agricultural settlements in the country. Grains and cotton were grown there thousands of years ago and a variety of domestic animals were reared. These natural advantages gave rise to a high population density even in prehistoric times.
Loess is a major factor in agriculture all over the world, not only in China. It is particularly well suited to the cultivation of grains. The wheat fields of the Ukraine, and those of Argentina and the U.S. are outstanding examples of the value of loess based soils. Knowing how easily the Yellow River can erode loess, Chinese authorities have terraced steep slopes and planted trees on the flat areas thus created. The trees tend to hold the soil in place during heavy rain.
In order to gain greater control of the problems associated with loess, research was undertaken by the Chinese government in the last quarter of the twentieth century. Engineers from China and Europe studied bedrock geology, quaternary sediments, geomorphology, landslide distribution, land use, and geotechnical properties of loess soil. Their findings made possible a much higher density of population and industrial installations in provinces like Shaanxi. A new problem arose as increasing numbers of people came to live and work there. As modern life impacted the traditional ways of Shaanxi, many workers, especially younger people, insisted on leaving the underground caves and building new homes above ground. In the new economy in which they were involved, these people had the financial resources to make changes. There had always been problems of dampness and darkness in the loess caves, especially in the ones that had to be excavated vertically into flat areas in contrast to those that had been dug into cliff faces. Former generations had no choice but to accept these limitations and even today there are millions who still live in the cave dwellings.
The 1556 Shaanxi earthquake was not the worst disaster in Chinese history. Millions have died from time to time when flooding destroyed their farms and they starved because there was no emergency aid from government sources. The Yellow River has sometimes been called the pride of China and sometimes its sorrow. Over the thousands of years past, it changed its course completely five times and its levees collapsed as many as 1,500 times. The yellow loess is carried along by the river and wherever there is an obstacle in the streambed it builds up and builds up over time until it overtops the levee and floods the adjacent farms. One and a half billion tons of loess in the form of silt is deposited annually near its mouth by the Yellow River. Often people tried to prevent the flooding that occurs by building small dams on the river but this only accelerates the problem of buildup. The 1887 flood, which is described elsewhere, killed more than a million people. In 1938, as China was being invaded by Japanese troops, the government of that time broke the levees in order to stop the advance of the soldiers. In the flooding that followed, 700,000 died. Even in modern times authorities seem unable to cope with the buildup of silt behind dams.
In the 1950s, in order to prevent flooding and at the same time generate electricity, Chinese engineers built a gigantic concrete dam, assuming that the silt, being of a fine texture, would flow through the turbines rather than create a buildup behind the dam. They were wrong. The same old problem reappeared. The plan was to have a lake of almost 100 billion units of water. Within two years, 20 percent of this amount had been lost to silting and gradually over the following eight years the total amount of water in the dam shrunk to seven units. The plan was scrapped. Alongside these problems of silting and flooding there is uncertainty about rainfall as has already been mentioned. In 2006, for instance, Beijing and its surrounding area had a serious shortage of rainfall and, at the same time, experienced recurring sandstorms. Attempts were made to seed a few clouds by sending tiny sticks of iodide high into the sky. A small amount of rain was obtained by this method.
